The relative merits of various cooling methods

Abstract

SUMMARY Three different methods of rapidly cooling specimens prior to microscopical analysis are currently in use, namely, slamming, plunging and spraying. The heat transfer processes and specimen cooling rates which are theoretically achievable by each method are discussed and compared with some of the published experimental data. For the slamming method it is concluded that very little improvement in specimen freezing will be achieved by pre‐cooling the block below about 20 K and where pure silver is the most efficient block material. The highest cooling rates and best microscopy by either the plunging or spraying method will be achieved using nitrogen at supercritical pressures. At subcritical pressures propane will produce the best results for metal samples whereas ethane is superior to propane for exposed biological specimens. Future experimental work which relates ice crystal size to cooling rate must be based on the cooling rate at the instant of freezing or nucleation and not on some average cooling rate.